1. Field of the Invention
The present invention relates to a multilayer inductor.
2. Description of the Related Technology
Multilayer inductors contain magnetic ceramic layers and conductive layers, which are stacked to form a helical conductive coil in the magnetic ceramic material. When a direct current is applied to a multilayer inductor at a certain level, the inductance of the multilayer inductor is reduced due to magnetic saturation. This phenomenon can be improved by modifying a closed magnetic path type multilayer inductor into an open magnetic path type, specifically by, as shown in FIG. 17, placing a nonmagnetic insulating layer 103 between magnetic layers 101 in a laminate as described in JP-A-56-155516.
Further, a method of improving a direct current superposition property by, as shown in FIG. 18, placing a nonmagnetic insulating ceramic 203 on at least a part of a magnetic ceramic 201 in a coil 202 is proposed in JP-A-11-97245.
However, a multilayer inductor according to JP-A-56-155516, which contains the nonmagnetic insulating layer between the magnetic layers, is disadvantageous in that the nonmagnetic insulating layer separates the magnetic path inside or outside the multilayer inductor, to greatly reduce the inductance value. In an inductor according to JP-A-11-97245, which contains the nonmagnetic insulating ceramic on at least a part of the magnetic ceramic in the coil, the magnetic flux density is higher in a contact region of a conductive layer forming the coil and the nonmagnetic insulating ceramic than at the center of a magnetic ceramic region surrounded by the coil. In a case where the nonmagnetic insulating ceramic has a small thickness, the conductive layer forming the coil is in unstable contact with the nonmagnetic insulating ceramic, whereby the nonmagnetic insulating ceramic can prevent the passing of the magnetic flux only nonuniformly. Thus, when a direct current is applied to the inductor, the inductance value is rapidly reduced without improving the direct current superposition property in 10 to 30% of such inductors. On the other hand, in a case where the nonmagnetic insulating ceramic has a large thickness to prevent the nonuniformity, the nonmagnetic insulating ceramic separates the magnetic path of the multilayer inductor to greatly reduce the inductance value, as with JP-A-56-155516.